Internal combustion engine fueling, exhaust gas recirculation and canister purge control require an accurate measure of the rate at which fresh air is being ingested into the engine cylinders. “Mass airflow” as it is commonly referred to must be determined in order that the air/fuel ratio be controlled to a predetermined ratio in accordance with well known performance and emissions objectives. This is true whether the fuel is metered to individual cylinders such as with well known port fuel injection or with single point fuel injection, the former requiring even more stringent requirements in the accuracy and responsiveness of the mass airflow estimates. Generally, it has been desirable to control the amount of fuel metered such that a stoichiometric ratio of 14.6/1 air to fuel is achieved. This has been primarily due to emissions considerations in modern automobiles which employ three-way catalytic converters for treating undesirable exhaust gas constituents. The stoichiometric fuel/air ratio results in little or no oxygen in the exhaust gas that is recirculated in the exhaust gas recirculation (EGR) system. Thus, traditional port flow models generally ignore the contribution of oxygen in the exhaust gas that is recirculated through the EGR system.
Advances in engine control technology and exhaust gas treatment technology have increased the use of lean-burn engines which operate at non-stoichiometric (e.g., higher) air/fuel ratios. In lean-burn engines the amount of oxygen in the exhaust gas is significant. Therefore, traditional port flow models may incorrectly predict the effective in-cylinder air to fuel ratio significantly. For example, where the intake port airflow comprises 40% returned through the EGR system, using a lean-burn air/fuel ratio of 30/1 can result in an inaccurate port flow estimate and fuel input, resulting in an actual in-cylinder air/fuel ratio of 40/1. Since lean-burn engines typically require even more precise control of the air/fuel ratio than traditional engines, the failure of traditional port flow models to accurately predict the mass airflow associated with the EGR has a significant impact on the performance of these engines.
Therefore, it is desirable to establish a method of determining intake port mass airflow output that is suitable for use with lean-burn internal combustion engines and that is adapted to accurately estimate the intake port airflow output to the combustion chamber for both stoichiometric and non-stoichiometric operating conditions.